Non-contacting tension measurement for molten films

A novel air-nozzle system was developed to perform non-contacting measurement of film tension in molten films. The measurement is based on correlation between the film tension and the pressure drop across the gap from the nozzle tip to the film. Films with higher tension exhibit less deflection by the impinging air jet and, consequently, produce a higher pressure drop across a smaller gap. The correlation between film tension and pressure drop was obtained with solid films at room temperature. A procedure was developed to extend the correlation to higher temperatures. The correlation was also found to agree with film tension versus pressure drop data obtained from specially designed molten film experiments. The capability of the system was demonstrated in the film casting of a linear low-density polyethylene melt and a low-density polyethylene melt. Tensions in the molten films were measured at various values of draw ratio and extrusion speed.